Lobeline (.alpha.-lobeline) is a lipophilic, non-pyridino, alkaloidal constituent of Indian tobacco (Lobelia inflata). As shown by the following formulas, no obvious structural resemblance to S(-)nicotine is apparent: ##STR1##
Structure-function relationships between nicotine and lobeline do not suggest a common pharmacophore (Barlow and Johnson, 1989). Nonetheless, lobeline has been reported to have many nicotine-like effects including tachycardia and hypertension (Olin et al., 1995), bradycardia and hypotension in urethane and pentobarbital anesthetized rats (Sloan et al., 1988), hyperalgesia (Hamann and Martin, 1994), anxiolytic activity (Brioni et al., 1993), and improvement of learning and memory (Decker et al., 1993). Moreover, lobeline has been used as a substitution therapy for tobacco smoking cessation (Nunn-Thompson and Simon, 1989; Prignot, 1989; Olin et al., 1995); however, its effectiveness is controversial as reflected by both positive (Dorsey, 1936; Kalyuzhnyy, 1968) and negative reports (Wright and Littauer, 1937; Nunn-Thompson and Simon, 1989). Furthermore, only short-term usage of lobeline as a smoking deterrent has been recommended due to its acute toxicity (nausea, severe heartburn and dizziness) and the lack of information concerning its long-term usage (Wright and Littauer, 1937; Olin et al., 1995).
In behavioral studies, nicotine has been shown to increase locomotor activity (Clarke and Kumar, 1983a, 1983b; Clarke, 1990; Fung and Lau, 1988), and to produce conditioned place preference (Shoaib et al., 1984); Fudala et al., 1985) in rats. However, the results of the latter studies are controversial (Clarke and Fibiger, 1987). In contrast, lobeline does not increase locomotor activity (Stolerman et al., 1995) or produce conditioned place preference (Fudala and Iwamoto, 1986). Although initially lobeline was shown to generalize to nicotine in discrimination studies (Geller et al., 1971), most subsequent studies have failed to reproduce this original finding (Schechter and Rosecrans, 1972; Reavill et al., 1990; Romano and Goldstein, 1980).
Nicotine has been reported to be avidly self-administered by rats (Corrigal et al. 1992, 1994; Donny et al., 1996); however, the ability of lobeline to support self-administration has not been investigated. Based on the differential effects of lobeline and nicotine in behavioral studies, it appears that these drugs may not be acting via a common CNS mechanism, even though lobeline is often considered to be a nicotinic agonist (Decker et al., 1995).
The positive reinforcing effect of nicotine is believed to be due to the activation of central dopaminergic systems (Bowell and Balfour, 1992; Corrigal et al., 1992, 1994). Presynaptic nicotinic receptors have been found on dopamine (DA)-containing nerve terminals (Giorguieff-Chesselet et al., 1979; Clarke and Pert, 1985). Nicotine binds to nicotinic receptors with high affinity (Kd=1-7 nM) (Lippiello and Fernandes, 1986; Reavill et al., 1988; Romm et al., 1990; Bhat et al., 1991; Loiacono et al., 1993; Anderson and Arneric, 1994). Also, lobeline has been reported to displace [.sup.3 H]nicotine binding from central nicotinic receptors with high affinity (Ki=5-30 nM) (Yamada et al., 1985; Lippiello and Fernades, 1986; Banerjee and Abood, 1989; Broussolle et al., 1989).
Chronic treatment with nicotine results in an increase in the number of nicotinic receptors in many regions of rat and mouse brain (Collins et al., 1990; Bhat et al., 1991, 1994; Marks et al., 1992; Sanderson et al., 1993). An increase in the number of nicotinic receptors in postmortem human brain tissue obtained from smokers also has been reported (Benwell et al., 1988). In contrast, chronic lobeline administration did not increase the number of nicotinic receptors in mouse brain regions in which increases were observed following chronic nicotine administration (Bhat et al., 1991).
Nicotine evokes DA release in in vitro superfusion studies using striatal slices (Westfall, 1974; Giorguieff-Chesselet et al., 1979; Westfall et al., 1987; Harsing et al., 1992) and striatal synaptosomes (Chesselet, 1984; Rowell et al., 1987; Rapier et al., 1988, 1990; Grady et al., 1992, 1994; Rowell and Hillebrand, 1992, 1994; Rowell, 1995), and in in vivo studies using microdialysis in striatum (Imperato et al., 1986; Damsma et al., 1989; Brazell et al., 1990; Toth et al, 1992). Nicotine-evoked DA release is calcium-dependent, mecamylamine-sensitive and mediated by nicotinic receptors (Giorguieff-Chesselet et al., 1979; Westfall et al, 1987; Rapier et al., 1988; Grady et al., 1992). Mecamylamine is a noncompetitive nicotinic receptor antagonist, which more effectively blocks the ion channel of the receptor (Varanda et al., 1985; Loiacono et al., 1993; Peng et al., 1994). Similar to nicotine, lobeline has been reported to increase DA release from superfused rat and mouse striatal synaptosomes (Sakurai et al., 1982; Takano et al, 1983; Grady et al., 1992). Based on these neurochemical studies, lobeline was suggested to be an agonist at nicotinic receptors (Decker et al., 1995). It is difficult to reconcile that nicotine and lobeline similarly release DA and displace [.sup.3 H]nicotine binding; however, the observed upregulation of nicotinic receptors following chronic nicotine administration is not observed following chronic lobeline administration.
Earlier studies of the pharmacokinetic properties of lobeline have centered on its proposed use in the treatment of nicotinism. For example, U.S. Pat. Nos. 5,536,503; 5,486,362; 5,403,595; and PCT Publication WO 92/19241 are all related to a drug delivery system and method for treating nicotine dependence. U.S. Pat. Nos. 5,414,005; 4,971,079; and 3,901,248 also discuss the use of lobeline in the context of treating nicotine abuse and/or addiction. A scientific article has studied the actions of morphine, lobeline, and other drugs in inducing "analgesia" in rats (S. Hamann et al. 1994). However, these workers did not equate their finding of an "analgesic" response for lobeline to a reduction of the pain response in man, nor did they propose the use of lobeline in treating drug abuse, withdrawal from addiction, and the like.
Similarly, to the present inventors' knowledge, the use of lobeline in the treatment of eating disorders has not been proposed. This is in spite of the widely accepted ability of nicotine to suppress appetite (see, e.g., Remington's Pharm. Sci., 18th ed., p.891) and the previously proposed association of obesity with reduced bioavailability of dopamine (U.S. Pat. Nos. 5,552,429; 5,576,321; 5,272,144; and 5,468,755).
The present study further elucidates the mechanism of action of lobeline using [.sup.3 H]dihydrotetrabenazine ([.sup.3 H]DTBZ), a structural analog of tetrabenazine (TBZ), which binds to a single class of high-affinity sites on the vesicular monoamine transporter-2 protein (VMAT2) to inhibit vesicular DA uptake (Pletscher et al., 1962; Scherman et al., 1986; Kilbourn et al., 1995; Liu et al., 1996). Of note, TBZ does not alter spontaneous efflux of [.sup.3 H]DA from rat brain vesicles (Floor et al., 1995). Taken together, TBZ appears to block [.sup.3 H]DA uptake into vesicles but does not promote [.sup.3 H]DA release from vesicles.
In the present study, the effect of lobeline is compared with that of d-amphetamine, a psychostimulant and lipophilic weak base reported to inhibit DA uptake into striatal synaptic vesicles (Philippu and Beyer, 1973; Ary and Komiskey, 1980) and to inhibit monoamine uptake into human VMAT2 expressed in CV-1 cells (Erickson et al., 1996). d-Amphetamine has also been reported to release DA from synaptic vesicles of the Planorbis corneus giant DA cell, increasing DA concentrations in the cytosol and promoting reverse transport of DA via DAT (Sulzer and Rayport, 1990; Sulzer et al., 1995). Furthermore, d-amphetamine has been reported to inhibit [.sup.3 H]DTBZ binding to rat striatal homogenates (Rostene et al., 1992) and human VMAT2 expresed in COS cells (Gonzalez et al., 1994), but with low potency. The ability of lobeline to evoke [.sup.3 H]DA release from rat striatal synaptic vesicles preloaded with [.sup.3 H]DA is also assessed in the present study.